Detection or sensing systems have been developed for use with various kinds of manufacturing equipment and power tools. Such detection systems are operable to trigger a reaction device by detecting or sensing the proximity or contact of some appendage of an operator with some part of the equipment. For example, existing capacitive contact sensing systems in table saws detect contact between the operator and the blade.
FIG. 1 depicts a prior art capacitive sensing based detection system 90 that is incorporated with a table saw 1. The detection system 90 drives an excitation voltage that is electrically coupled to a movable blade 22 of the saw 1, and detects the current drawn from the blade 22. The amplitude or phase of the detected current and/or excitation voltage changes when the blade 22 comes into contact with an electrically conductive object (such as an operator's hand, finger or other body part, as well as work pieces). The characteristics of the changes are used to trigger the operation of a reaction system 92. The reaction system 92 disables operation of the blade 22 by, for example, applying a brake to cease motion of the blade 22 and/or by dropping or otherwise removing the blade 22 from the cutting area. One example of a reaction system 92 uses an explosive charge to drive a stopper (not shown) into the blade 22 to arrest the motion of the blade 22. In addition, or instead, an embodiment of the reaction system 92 drops or collapses a blade support member (not show) to urge the blade 22 below the surface of the table 14.
The embodiment of the detection system 90 shown in FIG. 1 includes an oscillator 10 that generates a time-varying signal on line 12. The time-varying signal is any suitable signal type including, for example, a sine wave, a sum of multiple sine waves, a chirp waveform, a noise signal, etc. The frequency of the signal is chosen to enable a detection system to distinguish between contact with the first object, such as a finger or hand, and a second object, such as wood or other material, to be cut by the power tool. In the embodiment of FIG. 1, the frequency is 1.22 MHz, but other frequencies can also be used, as well as non-sinusoidal wave shapes. The oscillator 10 is referenced to the saw table 14 or other metallic structure as a local ground. As shown in FIG. 1, the blade 22 is disposed vertically in an opening defined by the saw table 14 (or work surface or cutting surface or platform).
The oscillator 10 is connected to two voltage amplifiers or buffers 16, 18 through the line 12. The first voltage amplifier 16 has an output connected to line 20, which operatively couples the output of the oscillator to the saw blade 22. A current sensor 24 operatively couples a signal from line 20 onto line 26 that is fed to an amplifier 28, which is connected to a processor 30 by line 32. The current sensor 24 is, for example, a current sense transformer, a current sense resistor, a Hall Effect current sense device, or other suitable type of current sensor. An output line 34 from the processor 30 is operatively connected to the reaction system 92 so that the processor 30 triggers the reaction system 92 if predetermined conditions are detected indicating, for example, contact between the blade 22 and the first object.
The signal on line 26 is indicative of the instantaneous current drawn by the blade 22. Because the saw blade 22 is in motion during operation of the table saw, the connection is made through an excitation plate 36, which is mounted generally parallel to the blade 22. The plate 36 is driven by the first voltage amplifier 16, and is configured with a capacitance of approximately 100 picoFarad (pF) relative to the blade 22 in the embodiment of FIG. 1. The plate 36 is held in a stable position relative to the side of the blade 22. The excitation plate 36 is configured to follow the blade 22 as the height and bevel angle of the blade 22 are adjusted during operation of the saw 1.
The capacitance between the first object and the saw table 14 (or power line ground if one is present) is in the range of approximately 30-50 pF in the embodiment of FIG. 1. When the capacitance between the excitation plate 36 and the saw blade 22 exceeds the capacitance between the first object and the saw table 14, the detection thresholds are not unduly affected by changes in the plate-to-blade capacitance. In the configuration of FIG. 1, the plate 36 is arranged in parallel with the blade 22 on the side where the blade 22 rests against the arbor 37, so that changes in blade thickness do not affect the clearance between the blade 22 and the plate 36. Other methods of excitation, including contact through the arbor bearings or brush contact with the shaft or the blade, could be used to the same effect.
In the detection system 90, the second-amplifier 18 is connected to a shield 38, and the amplifier 18 drives the shield 38 to the same potential as the excitation plate 36. Also, sensors in the detection system 90 optionally monitor the level of electrical current drawn by the shield 38. The shield 38 extends around the blade 22 underneath the table 14, and is spaced some distance away from the blade 22 on the top of the table 14 in the configuration of FIG. 1. The configuration of the shield 38 reduces the static capacitance between the blade 22 and the table 14, which acts as a ground plane if the table is not electrically connected to an earth ground. In various embodiments, the shield 38 is a continuous pocket of mesh, or some other type of guard that is electrically equivalent to a Faraday cage at the excitation frequencies generated by the oscillator 10. The shield 38 optionally includes a component that moves with the blade adjustments, or is large enough to accommodate the blade's adjustment as well as the various blades that fitted on the table saw. In the configuration of FIG. 1, the shield 38 moves with the blade adjustments, and includes a throat plate area of the table top 14.
The processor 30 performs various pre-processing steps and implements an adaptive trigger that enables detection of conditions indicative of contact between the first object and the blade 22. The processor 30 optionally includes one or more associated analog-to-digital (A/D) converters. The blade current signal from the current sensor 24 is directed to one or more of the A/D converters, which generate a corresponding digital signal. A blade voltage signal representing the voltage drop between the blade 22 and the excitation plate 36 is directed an A/D converter to generate a digital blade voltage signal in some embodiments. The processor 30 receives the digitized signal and performs various digital signal processing operations and/or computes derivative parameters based on the received signal. The processor 30 analyzes or otherwise performs operations on the conditioned blade signal to detect conditions indicative of contact between the first object and the blade 22.
Existing detection systems, such as the system 90 of FIG. 1, are configured for use when the power tool is operating to move an implement. For example, in the saw 1, the detection system 90 identifies contact between the moving saw blade and an object based on a transient signal, such as a spike in the digital blade voltage signal. The transient signal occurs over a short time, but system 90 recognizes the signal and the processor 30 takes appropriate actions within a brief time period of the contact between the blade and the object. When the saw blade is not moving, however, the transient signal still occurs if the signal is being passed through the blade, but the signal quickly returns to a similar level from before the contact occurred, and the detection system 90 cannot effectively identify long-term contact between the object and the implement. Additionally, if the object is already in contact with the static implement when the oscillator 10 begins generating the signal, then the detection system 90 does not detect the transient signal corresponding to initial contact between the object and the implement. For example, if an operator holds the saw blade 22 while the saw blade 22 is not moving, the sensing system 90 cannot identify the prolonged contact between the hand of the operator and the static blade 22. Consequently, improvements to power tools that enable identification of contact between an object and a non-moving implement in the power tool would be beneficial.